1. Field of the Invention
The present invention relates to a fixing apparatus favorably for use in an electrophotographic image forming apparatus, and to an image forming apparatus having the same.
2. Description of the Related Art
In an image formation using an electrophotographic system, a photoreceptor charged with a uniform electric potential is exposed to light in accordance with image information so that an electrostatic latent image is formed. The formed electrostatic latent image is developed by a developer so as to be visualized. The visualized image is transferred on a recording paper or the like, and the transferred developer on the recording paper is made to be fixed so as to form a solid recording image.
The fixing apparatus used for such image formation, is generally composed of a heating roller and a pressure roller, which are such configured that in passing the recording paper on which the developer for forming a visualized image through a pressure contact region (hereinafter referred to as a nip section) of the heating roller and the pressure roller which is formed by pressing the pressure roller against the heating roller, unfixed developer is fused and fixed by heating of the heating roller and pressing of the pressure roller.
During a fixing operation in the fixing apparatus, there sometimes occurs a so-called hot offset that the developer fused on the nip section of the both rollers is not all fixed on the recording paper, but a part of the developer is attached to a surface of the roller. For instance, the developer attached to the heating roller is transferred on a portion which should be properly a white base, on a recording paper on which the developer is to be subsequently fixed, with the result that an image defect is made to occur.
Moreover, on the pressure roller, the developer which has already fixed to a back surface of the conveyed recording paper, for instance as in a case of duplex print, is sometimes fused again by heat in passing through the nip section and a part of the developer is transferred and attached to the pressure roller. The developer thus attached to the pressure roller may cause the image defect and further, may cause a soil of the back surface of the recording paper.
The image defect caused by the hot offset in the fixing apparatus sometimes remains, in a case of black-and-white print, mere defects such as a fog in a white base of the formed image, a soil on the back surface of the recording paper, or the like in a tolerable range. However, in a case of full-color print, since a developer having a color different from a prescribed one is transferred from the both rollers, there often occur practically intolerable defects.
As a related art for solving such a problem, there is an apparatus having roller cleaning means on the both rollers provided in a fixing apparatus (refer to Japanese Unexamined Patent Publication JP-A 2003-107952).
FIG. 9 is a schematic view showing a configuration of roller cleaning means 1 provided in a related art fixing apparatus. FIGS. 10A to 10C are views for explaining a general outline of operation of the roller cleaning means 1 shown in FIG. 9. FIG. 9 illustrates the roller cleaning means 1 provided on a heating roller 2 in the fixing apparatus.
The roller cleaning means 1 comprises a feeding roller 4 for feeding a belt-shaped cleaning member 3 which has been previously rolled up, a winding roller 5 for taking up the cleaning member 3 fed from the feeding roller 4, and a pressure-contact roller 6 (also referred to as a web pressure-contact roller) provided between the feeding roller 4 and the winding roller 5 so as to press the cleaning member 3 on the heating roller 2.
The roller cleaning means 1 presses the cleaning member 3 on the heating roller 2 which is rotated in an arrow sign 7 direction in a state where the winding roller 5, the feeding roller 4, and the pressure-contact roller 6 are made to be at rest without being rotated so that the heating roller 2 and the cleaning member 3 are made to be slidingly scrubbed. By so doing, a developer 8a attached in a fused state to an outer circumferential surface of the heating roller 2 is removed and the removed developer 8b is accumulated, still in a substantially fused state, in a gap formed by the cleaning member 3 located between the pressure-contact roller 6 and the winding roller 5, and the surface of the heating roller 2.
When the developer 8b accumulated in the gap reaches a certain amount level, the roller cleaning means 1 operates the winding roller 5 for take-up in an arrow sign 9 direction so as to take up the cleaning member 3 only by a predetermined length, with the result that the developer 8b is made to detach from the surface of the heating roller 2 in a state where the developer 8b is attached to the cleaning member 3.
Hereinafter, an operation of the roller cleaning means 1 in the related art will be further described with reference to FIGS. 10A to 10C. FIG. 10A shows a state where the above-described winding and feeding rollers 4, 5 and the pressure-contact roller 6 are in a rest state, and the developer 8a on the surface of the heating roller 2 is removed by the cleaning member 3 and accumulated in the gap between the cleaning member 3 and the surface of the heating roller 2.
FIG. 10B shows a state where an amount of the developer 8b accumulated in the gap reaches a certain level, and the winding roller 5 is made to be rotated so as to start taking-up of the cleaning member 3. When the cleaning member 3 is taken up in an arrow sign 10 direction by driving the winding roller 5, the pressure-contact roller 6 is also rotated by frictional force acting between the pressure-contact roller 6 and the to-be-taken-up cleaning member 3. When the pressure-contact roller 6 is rotationally activated, attributable to actions such as the frictional force between the pressure-contact roller 6 and the cleaning member 3, and adhesion of the fused developer 8b impregnated in a minute air gap of the cleaning member 3 formed by, for instance, Nomex paper (trade name), there sometimes arises a phenomenon that the cleaning member 3 moves as being attached to the pressure-contact roller 6, namely that the cleaning member 3 is taken up by the pressure-contact roller 6.
FIG. 10C shows a state where the winding roller 5 has further rotated so that a taken-up tension in the arrow sign 10 direction for the cleaning member 3 has further increased. When the taken-up tension further increases and force acting in a direction moving away from an outer circumferential surface of the pressure-contact roller 6 for the cleaning member 3 exceeds an adherence of the cleaning member 3 to the pressure-contact roller 6, the cleaning member 3 is rapidly detached from the surface of the pressure-contact roller 6. By so doing, the cleaning member 3 pulsates in a direction perpendicular to a taken-up direction 10 and in a direction moving close to and away from the surface of the heating roller 2 (an arrow sign 12 direction). Since force generated by pulsation of the cleaning member 3 acts the developer 8b in a fused state, attached to the cleaning member 3, the developer 8b is dispersed and attached again to the surface of the heating roller 2.
Consequently, in the roller cleaning means 1 of the related art, there arises a problem that a valuable cleaning effect diminishes since the developer 8b removed from the surface of the heating roller 2 by the cleaning member 3 is attached again to the heating roller 2 by the pulsation of the cleaning member 3 at the time of starting the taking-up operation for taking up the cleaning member 3.
Moreover, in a cleaning method using such a cleaning member 3 in the related art, it is required to replace a portion of the cleaning member 3 used for cleaning when the developer 8b has been accumulated to a certain extent in the gap between the cleaning member 3 and the surface of the heating roller 2, in other words when the cleaning member 3 is soiled by the developer 8b. There arises another problem that a time for replacement is not made clear in the related art in spite of a fact that its cycle for replacement varies depending on a print image density, a print image size, or the like.